A conventional technique for observing the subsurface subsidence phenomenon is described in the article by R. D. Allen, entitled "Collar and Radioactive Bullet Logging for Subsidence Monitoring", Tenth Annual Logging Symposium, Society of Professional Well Log Analysts, May 1969, herein incorporated by reference. The article discusses implanting a plurality of axially-spaced radioactive bullets in the side walls of a borehole, and periodically recording nuclear well logs by means of a well logging tool having two longitudinally-spaced scintillation gamma ray detectors so as to detect any changes that might have occurred in the distances between the bullets, thereby evaluating the magnitude of the subsidence phenomenon.
When acquiring a log, gamma ray intensity is recorded continuously as a function of information relating to the depth of the logging tool as provided by a tangential wheel device which is directly engaged with the cable. Given the elasticity of the cable and the somewhat irregular displacement of the logging tool suspended at the end of the cable, the resolution of depth measurements is not sufficient for observing very small variations as a function of time.
The present invention provides a solution to these drawbacks and makes it possible to substantially improve the precision with which the distance separating the radioactive markers is measured.